October 27, 2017
Initially developed for military purposes, the .308 Winchester is one of our finest sporting cartridges.
In 1952 Winchester had a new .30-caliber, U.S. military service cartridge in the works, optimized for semiautomatic and full-automatic firearms and internally called the "T65." The company released the cartridge as a commercial product, the .308 Winchester, well before the United States adopted the military version that we now know as the 7.62x51mm NATO.
Optimization for fast-firing military guns affected cartridge case length and cartridgeâ€¨overall length (COL). The final T65 incarnation was roughly a half-inch shorter than the .30-06. Sport shooters, awash in a trend toward bigger case capacities, raised some eyebrows at the compact .308 Win. The .308 Win. looked very much like the old .300 Savage fitted with a longer neck. Sources cite the .300 Savage as the 7.62's parent. That makes sense because the first experimental T65 variant used a 47mm case, just like the Savage cartridge.
"Smaller case, less performance" was a common mind-set in 1952, but the compact 7.62mm com- pared favorably to mil-spec .30-06 performance. Ball propellants and a slightly higher pressure limit than the .30-06 helped. For military ammo, the original 147-grain FMJ bullets in the 7.62mm M80 and 150-grain .30-06 M2 Ball ammo posted similar velocities at around 2,750 fps.
.308 Winchester Vs. .30-06
Although published factory ammo specs for the two cartridges show very similar velocities, does this parity extend to handloaded ammo? Not easily. Remember, factories have access to many more propellants than handloaders. I have lab data shot for pressure and velocity in .308 and .30-06 under identical conditions and fired only days apart with the same bullet (an experimental 150-grain bullet) and propellant lots. The average velocity advantage to the .30-06 is 156 fps. On average the .30-06 required a 14.2 percent propellant charge increase to post an average 5.6 percent velocity increase.
In short, the handloaded .308 may equal and sometimes exceed .30-06 velocities with bullets of 130 grains and less; the .30-06 has the advantage with 180-grainers and up. In general, the .308 Win. shines with the midrate propellants, and its larger cousin prefers its fuels be a bit slower burning.
Did soldiers view the smaller cartridge as a back- step? Judging from old gun magazine articles and letters from disgruntled frontline soldiers, it's possible. However, I was privy to data while working at the Crime Lab that showed a compelling advantage of the FMJ 7.62mm NATO military cartridge: terminal ballistics.
The .308's Advantage
I was fortunate to extensively inter- view a military professional who worked in a wound ballistics facility. He said that the 7.62mm — loaded with the same mil-spec bullet and fired into 15cm of ordnance gelatin — produced a temporary wound cavity almost twice the volume of the one produced by the .30- 06. I saw his data yet still wondered how this could be.
It was something simple: rifling twist rate. The .30-06 military rate was one turn in 10 inches, or 1:10. The 7.62mm was 1:12 and thus rotated more slowly in flight. We know it takes a certain minimum rotation rate to keep a bullet nose-on and stable through air. When the bullet makes the transition from air to something else, such as tissue, the resistance of the tougher medium slows the bullet faster. To stay nose-on in denser medium, the bullet needs a faster spin rate to overcome the effects of slowing on stability. When the spin can't match the speed loss, the bullet tumbles.
Any bullet fired into sufficient length of soft media will eventually tumble. If a bullet is very stable in tissue, it may completely penetrate its intended target and not tumble until it is several feet from the exit point. In the early wound ballistic tests my source conducted, only cavity formation within the first 15cm (roughly 6 inches) of gelatin was considered. The developers felt 15cm represented the typical front-to-back distance of the vital zone in a human chest.
The original 7.62mm NATO had a perfect combination of bullet shape, velocity, and twist for this scenario. The 1:12 twist was accurate in flight but went unstable in tissue much earlier than the .30-06 loaded with the same bullet at higher rotational velocity. That meant a much bigger wound cavity starting closer to the entrance site due to a bullet spinning just a little more slowly.
Did the 7.62mm developers realize this when working up the new cartridge in 1952? I can't say. Imaging technology not available in 1952 allowed my source to make direct comparison of cavity location and volume. However, changing wounding characteristics of non-expanding bullets by using induced tumbling was well known when 7.62mm development began.
The British 303 Mk VII rifle cartridge, adopted in 1910 and used long past World War II, had a 174-grain FMJ spitzer bullet that was almost as long as a .30-caliber 220-grain RN bullet. There was a lightweight filler plug between the lead core and the nose portion of the jacket that added length, resulting in reduced stability and quicker tumbling after impact. The "official" story is that the extra bullet length was needed so very early Enfield rifles throated for the original 215-grain FMJ RN could remain accurate with newer spitzer bullet ammo. Again, accidental or intentional — who knows for sure? This extended to British handgun cartridges as well. Both of the British service revolver cartridges, the .455 Revolver Mk II and the .380 Revolver Mk II-Z, had very oddly shaped bullets with strange nose profiles and/or massive hollowbases. They tumbled in gelatin — I've seen it.
Its military origins aside, the .308 Win. is one of our finest sporting cartridges. It gets overshadowed by the bigger .30-calibers and "Magnums" but remains an accurate and dependable game-getter that doesn't beat you senseless. Today's bullets allow the .308 to cleanly take most non-dangerous North American game, and it is a reloader's dream. You can easily make ballistically consistent reduced loads for training recoil-shy new shooters or load a wad of tack-driving target loads for range day. I hope it is around a long time.